The Tectonic setting of the Colchester plutons, Southwest Arm, Green Bay, Newfoundland

Sayeed, Usman Ahmed (1970) The Tectonic setting of the Colchester plutons, Southwest Arm, Green Bay, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The area is underlain by Early Ordovician volcanic rocks and plutons of intermediate intrusive rocks and small bodies of serpentinised ultramafic rocks. These are unconformably overlain by red beds of the Silurian Springdale Group. -- The Lush's Bight Group consists of basic variolitic pillow lavas with minor interbeds of sedimentary and pyroclastic rocks. Dikes and sills of diabase, quartz porphyry and lamprophyre cut the volcanic rocks. The Group has been divided into two broad lithological assemblages, the Jackson's Cove Unit and the Birchy Cove Unit, on the basis of colour, size, shape and orientation of pillows, presence or absence of varioles, degrees of deformation and alteration, petrological and chemical characters. The relative ages of these two units are uncertain. -- Two epi-zonal plutons intrude the Lush's Bight volcanic rocks near Colchester Pond and Manful Point. The Colchester Pluton is a zoned pluton which from margin to core varies in composition from meladorite through quartz diorite to potash deficient granophyric granodiorite. Granophyric texture of the central zone is interpreted as due to simultaneous crystallisation of quartz and felspar, after emplacement of the pluton. The compositional zoning is also interpreted as a post-emplacement event. -- The Cooper's Cove Pluton, north of Manful Point, consists of a dioritic marginal zone and a quartz monzonite central zone. Contacts are sharp and the quartz monzonite intrudes the diorite. The pluton originated by successive intrusions. -- The compositional resemblances of all the plutonic rocks in the area to the Lush's Bight basalts suggest a possible genetic relationship. The concept of an Island Arc and/or ocean floor environment seems to constitute the most likely milieu in which to produce these thick Ordovician pillow lava sequences and the various plutons and associated ultramafic pods. -- Rocks of this area have undergone two main phases of deformation, D₁ and D₂. The first resulted in tight F₁ folds with axes striking nearly due east; the second deformation, D₂, produced large scale F₂ open flexures with axes striking from north northeast to northeast. A third minor deformation, D₃, produced only kink bands which are best seen in the sheared meta-basalts. -- The sulphide mineralisation occurring in the area is interpreted as structurally controlled and epigenetic.

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